This invention relates to a packet re-transmitting system for a satellite communication system of the slotted-ALOHA type, and more particularly to a packet re-transmitting system for a satellite communication system of the slotted-ALOHA type wherein communications between a central office and a large number of local offices are effected by way of a communications satellite.
A satellite communication system of the slotted-ALOHA type is suitable to efficiently effect communications between a central office having a high information processing capacity and a large number of scattered local offices which involve a small amount of information by way of satellite channels interconnecting them.
By the way, if it is permitted for a large number of local offices to transmit packets at an arbitrary point of time, then the transmission packets will overlap frequently with each other on satellite channels, and consequently, the central office cannot receive them correctly.
Therefore, according to the satellite communication system of the slotted-ALOHA type, the central office transmits a slot signal, and each of the local offices transmits a packet signal in synchronism with the slot signal, thereby preventing otherwise possible occurrence of an overlap of transmission packets.
Even with the satellite communication system of the slotted-ALOHA type, however, it sometimes occurs that different local offices transmit packets to the same slot. In this instance, the packets collide with each other, and accordingly, each of the local offices must re-transmit its packet.
Therefore, minimization of such a packet re-transmission time is demanded.
FIG. 1 illustrates a conventional packet re-transmitting system for a satellite communication system of the slotted-ALOHA type. Referring to FIG. 1, the packet re-transmitting system shown includes a communications satellite 3, a central office 100 and a large number of local offices as represented by local offices 200a and 200b.
The central office 100 includes a terminal equipment 101, a packet transmission processing section 102, a modulator 103, a transmitter/receiver 104, a demodulator 105 and a packet reception processing section 106.
The terminal equipment 101 effects inputting and outputting processing of original data (packets). The packet transmission processing section 102 effects transmitting processing of a packet of data from the terminal equipment 101. The modulator 103 effects modulating processing such as, for example, QPSK.
The transmitter/receiver 104 includes, though not shown, a transmitting section including an up converter and a high power amplifier, and a receiving section including a down converter and a low noise amplifier, and effects up converting processing and amplifying processing for a signal from the modulator 103 and also effects down converting processing and so forth for a reception signal. The demodulator 105 effects demodulating processing for a reception signal, and the packet reception processing section 106 effects receiving processing of a packet of demodulated data.
Each of the local offices 200a and 200b includes a terminal equipment 201, a packet transmission processing section 202, a modulator 203, a transmitter/receiver 204, a demodulator 205, a packet reception processing section 206 and a timer 207.
The terminal equipment 201, packet transmission processing section 202, modulator 203, transmitter/receiver 204, demodulating section 205 and packet reception processing section 206 have substantially similar functions to those of the terminal equipment 101, packet transmission processing section 102, modulator 103, transmitter/receiver 104, demodulator 105 and packet reception processing section 106 of the central office 100, respectively. The timer 207 is provided to manage the time for re-transmission of a packet.
In the satellite communication system, the central office 100 transmits a slot signal SLT of a predetermined period by way of a down channel f1 while any of the local offices 200a and 200b transmits a packet in synchronism with the slot signal SLT.
By the way, in such a system which adopts a system of the ALOHA type or the slotted-ALOHA type as the satellite communication system described above, the scale (in diameter of an antenna, transmitting output power and so forth) of the local offices 200a and 200b is usually so small that, for example, the local office 200a cannot directly receive a relay signal from the communications satellite 3 of a packet transmitted from the local office 200a itself. Accordingly, the local office 200a cannot directly find from a relay signal from the communications satellite 3 whether or not a transmission packet therefrom has collided with another transmission packet from another local office 200b, but can find, only when it receives a reception answer signal (notification that a packet has been received regularly) from the central office 100 in accordance with an ordinary communications protocol, that the transmission packet has been received regularly.
In the meantime, at the central office 100, if packets from the local offices 200a and 200b collide with each other, then the data of them will be destroyed, and consequently, the central office 100 cannot determine whether or not a local office has actually transmitted a packet to the central office 100 and naturally cannot send back a reception answer signal.
Therefore, it is a conventional practice to employ, on the local office side, a timer which operates, for example, for several seconds (refer to the timer 207 in FIG. 1) after it sends out a packet and to follow a procedure wherein the packet is re-transmitted if no reception answer signal is received from the central office within the time of the timer.
Accordingly, with the conventional packet re-transmitting system, when packets of different local offices collide with each other on satellite channels, no reception answer signal can be received from the central office as described above, and consequently, a very long period of time is required for re-transmission of a packet. As a result, the transmission efficiency is low and data to be re-transmitted must be kept for a long period of time. The problems are particularly remarkable when data are transmitted in a plurality of packets.